Differential amplifiers are well-known electronic devices for amplifying a voltage difference between two input signals. A differential amplifier usually comprises two electrical paths (“legs”) that are independently coupled to a voltage source at one end, and are together coupled to a voltage or current source at an opposite end. Each electrical path usually comprises a transistor element and a resistance element. Two input signals can be applied to the transistor elements, and an output signal can be extracted from each electrical path between its transistor element and resistance element. The two electrical paths are usually designed to substantially match each other, such that components common to both input signals are cancelled out and a voltage difference between the input signals is amplified.
FIG. 1 illustrates a prior art differential amplifier based on N-type metal-oxide-semiconductor (MOS) transistors. As shown, MOS transistor N1 and resistor R1 form a first electrical path while MOS transistor N2 and resistor R2 form a second electrical path. The two electrical paths are independently coupled to a voltage supply VDD at one end and are together coupled to a common current source I0 at an opposite end. Input signals Vin and Vinb are applied to the gates of the MOS transistors, and differential outputs Vout and Voutb are extracted from between the transistors and the resistors. This type of differential amplifier is often used in pre-amplification stages of high frequency sampling and detection circuits such as a pre-amplifier in input receivers of high-speed links or memory interfaces. Differential amplification in an input receiver can reduce the effect of offset and kickback of subsequent input sampling circuits. It can also reject common-mode input noise for the subsequent sampling circuits, improving the accuracy and speed of sampling operations. An input receiver may contain one or more stages of differential amplification followed by a regenerative latch. One simple example is shown in FIG. 2, wherein a differential amplifier-based pre-amplifier (Pre-Amp) samples input signals Vin and Vinb, and outputs signals to a latch (Latch) where they are sampled via a clock signal (CLK) and stored, and provided as complimentary output signals (Q, Qb).
A number of problems may affect the performance of a differential amplifier. For example, a differential amplifier's major characteristics, such as gain and output swings, usually vary with process, temperature and voltage conditions. Moreover, device mismatches in a differential pair can degrade the amplifier sensitivity to low-swing input signals, thus reducing the effective sampling time aperture of an input receiver. An example of reduced sampling time aperture due to gain degradation and offset is shown in FIG. 3. Especially in a differential amplifier for high-data-rate and low-voltage applications (e.g., high-speed memory devices), the effect of process, temperature and voltage variations, as well as input offset, can be highly detrimental. To battle the input offset, some prior art designs resort to capacitor trimming and auto zero cancellation techniques, both of which tend to result in decreased operating speed in addition to increased area and cost.
In view of the foregoing, it would be desirable to provide an improved differential amplifier which overcomes the above-described inadequacies and shortcomings.